The present invention relates to an optical fiber-dispersive compensating device utilizing non-degenerate fourwave mixing of a semiconductor laser.
In optical fiber communication, larger transmission speed and transmission distance of signals cause deterioration of signal waves due to frequency dispersion of a signal in an optical fiber forming a transmission path. This results in limiting the transmission speed or distance. For example, for the transmission of a light signal at 10 Gb/s using a 1.55 .mu.m wavelength signal using an ordinary 1.3 .mu.m zero-dispersion fiber, transmission distance is limited to several tens of km or less due to secondary frequency dispersion.
Several methods are available for compensating for the frequency dispersion. One of the most useful of these methods is a dispersive compensating method utilizing a non-linear optical effect since its compensation may be effected theoretically free of transmission distance to be compensated. In this method, a dispersion-compensating device containing a non-linear optical element is provided at the intermediate point of a transmission path having substantially uniform frequency dispersion. In the dispersion-compensating device, the spectrum of signal light deteriorated by transmitted frequency dispersion is frequency-converted about a frequency axis and fed back into the transmission path on the receiving side. This may compensate for frequency dispersion of the first half of the transmission path. For example, due to frequency dispersion of the optical fiber, higher frequency components of signal light may travel faster than lower frequency components. In this process, the signal waveforms deteriorate as the length of propagation in the optical fiber increases. If the frequency conversion of signal light as hereinbefore described is performed at the intermediate point on the transmission path, lower frequency components of signal light before conversion are converted into higher frequency components. Thus, portions which are delayed on the first half of the transmission path are no longer delayed as they reach the last half of the transmission path, thus compensating for the deterioration of the signal waveforms at the receiving side.
Four-wave mixing or light parametric amplification are used to achieve a non-linear optical effect. In this connection, for example, Henmi has filed a patent application of a device utilizing non-degenerate four-wave mixing of a semiconductor laser amplifier (Japanese Published Patent Application 1-263678, 1989). "Non-degenerate four-wave mixing" means that a pump light and a probe light (here corresponding to the input signal light) having a frequency different from that of the pump light are simultaneously incident on a non-linear optical medium. The output signal light contains spectra which include inverted probe light spectra on a frequency axis generated at symmetrical positions to the probe light relative to the pump light frequency.
The non-degenerate four-wave mixing of a semiconductor laser or a semiconductor laser amplifier is produced by two different mechanisms. One mechanism is due to carrier density modulation wherein frequency conversion is higher but frequency response is limited by the life time (about 1 ns) of a carrier. Therefore, signal light transmission speed to be converted is as low as about 1 Gb/s. Another mechanism is due to an intraband non-linear process which has a lower frequency conversion efficiency but exhibits a higher frequency response of several hundred GHz or more. Therefore, signals with 100 Gb/s or more may theoretically be converted. The latter mechanism, however, has very low frequency conversion efficiency. As a result, the former four-wave mixing mechanism caused by the carrier density modulation is widely used in conventional examples.
However, problems arise with the conventional examples. One problem is that, in order to utilize a non-linear optical effect, such as the four-wave mixing or the light parametric amplification, the pump light source requires a very large output. For example, utilization of optical fiber as a non-linear optical medium requires a pump light source of several watts or above. For this reason, laser devices, such as solid laser devices, are required for dispersion-compensating devices. However, it is difficult to use these devices for optical fiber transmission in view of their power consumption and stability. A very large output of the pump light source is necessary because of a lack of non-linear optical medium with high efficiency and capable of operating with a lower output pump light source of several tens of mW or below. The only exception is the use of the non-degenerate four-wave mixing of the semiconductor laser amplifier as described in the conventional example. In this case, a pump light source with 1 mW or less is used to obtain a high efficiency frequency conversion. However, because the conventional fourwave mixing caused by modulation of carrier density has a lower speed response, high speed signals with 1 Gb/s or above are difficult to frequency convert. If the four-wave mixing caused by the intraband nonlinear process is used, a higher speed signal must be converted. Because the conversion efficiency is extensively lower, a dispersive compensating device utilizing such phenomenon has not been realized to date.